The drying of products of the aforedescribed type plays an especially important role with comestibles, i.e. in the food industry, since an effective drying technique can eliminate the need for preservatives which might be detrimental to the health when ingested by certain individuals or when consumed in large quantities.
The earliest drying methods were, of course, relatively primitive, generally involving the exposure of the food product to conditions at which moisture tends to be lost from the product, e.g. by exposure to the sun, storage in ventilated or heated chambers, etc.
While such drying methods were found to be effective for many food products, they were incapable of effectively preserving others and hence modern technology recognizes a variety of more sophisticated drying techniques.
For example, freeze-drying and drying above the freezing point of the food product are both common for the preservation of foodstuffs of various types and in the production of "instant" products which can be restored to an edible form by the addition of water.
In freeze-drying, the product is introduced into a vacuum chamber and is subjected to subatmospheric pressures so that the drying results in a migration of the liquid substance from the food product by sublimation. In other drying techniques the moisture is lost primarily by evaporation.
In both of these systems the chamber is more or less continuously evacuated so that the released vapor or moisture is carried off with residual gases which may have been absorbed or retained in the product.
Such operations are not only uneconomical from an energy consumption point of view, but also may detrimentally affect the quality of the product. For example, in the freeze-drying of foods and like materials, the aromatic substances may be lost with the evacuated gases since such aromatics are relatively volatile. This is especially the case when upstream of the pump but downstream of the evacuated chamber, a low temperature zone is provided to enable the withdrawn substances to be collected.
In the dewatering or drying of biological materials, for example tissue samples, body fluids before or after separation into a multiplicity of components, pharmaceuticals, biological extracts and the like, a rapid drying can involve the danger that overheating may be destructive of cell walls or membranes and the protein content may be denatured; in general, there is the possibility that conventional techniques used in the drying of biological materials can cause a modification of the structure thereof or bring about other changes in the product which are undesirable.
It should be noted that such changes can also occur with freeze-drying, especially as to the cell structure.